For heating and cooking of food in pots and frying on pans many different heat sources are used, e.g. burning gas, incandescent spirals which may be built into steel plates or give off heat radiation, comprising halogen lamps, the light radiation of which is reflected from below against the bottom of the cooking pot. It is desired to have a source of heat which may be regulated as quickly as possible in order to obtain the most direct influence on the heating of the content in function of the instantaneous energy supply.
It has become common to supply the top of a hotplate or an assembly of hotplates with a coating which will endure the sliding of pots along its surface as well as resisting strong cleansing agents and consequently heat resistant glass is often used. The strong cleansing agents are necessary because a large part of the surface of an assembly becomes so hot that spilt food burns on easily. Furthermore it is essential that the sheet of glass has a considerable thickness because it must resist the sudden placement of heavy pots on the hotplate. The glass plate itself is carried by heavy brackets.
It has become usual to require that a unit for heating food shall have a low profile and a low temperature on the outside of the enclosure because the cooking range and oven are installed independently of each other and are built-in in separate locations. Furthermore it has become more and more common to use electronics for control of power and such electronic components are not durable at too elevated temperatures.
For industrial use induction heating is known in which the primary of a transformer is supplied with high frequency energy from a generator or transmitter and where the secondary wholly or in part consists of metal in a crucible. Since the content of the crucible is almost solid most of the high frequency energy will be dispersed as current losses by the eddy currents generated in the contents of the crucible which cannot run without the generation of heat because of the resistivity of the material. The penetration depth in the crucible is a.o. dependent on the frequency. When the term high frequency energy is used in this text it means energy transmitted at a frequency which exceeds ordinary power frequencies, in particular ultrasonic frequencies, i.e., above 20 kHz, however below 200 kHz.
It has proven advantageous to use the principles of induction heating for hotplates because the self-heating of the surface of such a hotplate may be very small. This is because ideally all of the energy is dispersed as eddy current losses in the bottom of the pot. The pot must have the double function of being ferromagnetic so that the lines of force are drawn into it and resistive so that the eddy currents will disperse energy. An iron cooking pot will fulfil this purpose well, but a copper pot would not. Magnetic lines of force which are not concentrated and will hence not traverse the bottom of the pot must be regarded as losses and standards for allowable radiation of electromagnetic energy request shielding of the magnetic field. Consequently it is usual to fit a network of a material which is highly magnetically conductive at the particular frequency below the field coil in order that the lines of force are as much as possible held between the magnetic shielding and the bottom of the cooking pot. Importantly it is also obtained that the sensitive electronic components are not subjected to the high frequency radiation. The network may be shaped as radial spokes of ferrite material.
It has been considered that such induction heating hotplates may advantageously be used in constructions using a thick, heat resistant sheet of glass as a top plate, because the magnetic lines of force may easily traverse air and glass.
However known induction heating hotplates have the disadvantage that the losses are too high which is displayed by a too high self-heating and the need for more expensive electronic components for the generation of the required high frequency energy. The mechanism is the following: in order to obtain a desired power in the pot energy must be transferred to overcome current losses in the coil and loss of density of magnetic lines of force at airgaps. Hence the current through the coil must be increased, and the heating of the enclosure is further increased. The network of magnetically conductive material used for increasing the field concentration is expensive because it is made of ferrite due to the high frequency. Ferrite is a composite of different metal oxides, among which a ferrous oxide, which has been sintered in an inert gas.
A consequence of the disadvantage discussed is that in known cooking ranges using a number of induction heating hotplates the different coils are inductively coupled with each other which may mean that under certain circumstances one may only be able to use one of the induction heating hotplates at a time.
A further disadvantage of the known induction hotplates is that the windings of the coils transmit ultrasonic energy at the driving frequency or its harmonics because of mechanical vibrations, and it may be perceived by house pets although humans remain unaffected.
It is a purpose of the invention to provide an induction heating hotplate which has considerably higher efficiency than known induction heating hotplates and hence much lower losses and radiation. This is obtained according to the invention in that the magnetic circuit is built like a transformer with a small airgap in which the cooking vessel acts as a yoke. When present text uses the expression airgap any interruption of the magnetic lines force is meant, where they h in non-magnetic material.
The apparatus for the heating of cooking vessels for food by means of electromagnetic induction according to a disclosed embodiment comprises a magnetic circuit constructed with a plate upon which is fitted a central pole piece and an outer ring in the same level above the plate, which parts are made in a material having low magnetic losses or eddy current losses at the frequency utilized. The coil is fitted into the ring-shaped space between the central pole piece and the outer ring.
A further feature of the invention enables simple facilities for changes in the supplied power in that one or several rings are fitted between the central pole piece and the outer ring and in that there is a coil in each space between rings.
It is a further purpose of the invention to provide a construction which is massively built in order that the wear resistant top layer may be quite thin. This is obtained by manufacturing all parts except the source of high frequency energy and the coil in one piece as an integrated block.
An advantageous fixation of the coil in the core is provided in that the coil is cast into the winding space into which it is fitted.
An advantageous material for a hotplate according to the invention is provided in that all parts, except the source of high frequency energy and the coil are manufactured in materials which are dimensionally stable and which are able to be cast.
A further advantageous combination of materials is provided according to the invention in that the magnetic circuit is made up of magnetic concrete and the support for the cooking vessel is cast-on of non-magnetic concrete, preferably of a DSP type.
A further advantageous and useful combination of materials is provided in that the support for the cooking vessel has a low thermic conductivity.
A construction is provided according to the invention which enables a pot dimension dependent supply of power to the pot while utilizing the transformer type construction. In this construction, the coils are combined and supplied with energy in dependence of the diameter of the bottom of the cooking vessel.
There is provided a prescription for the operation of the hotplate which allows electronic measurement of the pot dimension. Particularly, the apparatus has a mode of operation for supplying of power to a cooking vessel and another mode of operation with reduced power supply to the coils for measuring the degree of coupling between the cooking vessel and the magnetic core.
A further advantageous embodiment of the invention is provided for control in dependence of the pot and its placement in that in the measuring mode of operation the coils are connected in rotation in different combinations, and in that the output signal which constitutes an expression of the impedance which is determined by the diameter and placement of the cooking pot controls the combination of coils which is to be supplied with energy from the source of high frequency energy in the subsequent power transfer.
The invention will be described in greater detail in the following with reference to the drawing, where